Solar power has been viewed by many as a highly desirable energy resource, because it may be readily used to generate thermal and electrical energy. For example, a solar collector (usually formed by mirrors) may collect optical energy from the sun and direct the same to a transducer (receiver), which may convert the optical energy to either to thermal energy or electricity. The thermal energy is usually transport out (or between) of transducers to applicator via “heat transfer fluid” (HTF), e.g., such as water, oil and the like. By arranging solar collectors in arrays, power plants have been developed that may convert vast amounts of solar energy to energy used for desired applications.
In solar thermal applications, optical energy from the sun is converted to thermal energy for use in other applications, such as generating electrical energy employing known implements such as conventional turbine-electric generators or a Sterling Engine, or for cooling or heating. To that end, typically large arrays of individual solar modules (composed of optical collectors and thermal receivers i.e., the device for receiving, absorbing optical energy and converting it to thermal energy) are coupled by fluid pipes and transfer heat with HTF. Each module has a fixed power conversion and transfer capacity, i.e., that quantity of solar energy that may be collected and transferred to the thermal transfer fluid.
In such applications, thermal loss limits the overall conversion efficiency. Thermal loss is dominated by convection loss and “black-body radiation” loss (BRL). While convection loss can be reduced by thermally insulating the thermal receiver and HTF transfer pipes. However, (BRL) of the receiver is difficult to control. BRL is dependent upon receiver aperture area, temperature, and the material of the absorption surface of the collector. Specifically, BRL is linearly proportional to the receiving/radiating aperture area and to the 4th power of the temperature of the radiating body. In order to reduce BRL, and consequently, increase the overall conversion efficiency, it is desired to reduce the receiver area. One manner in which to reduce BRL while minimizing the inefficiency of the collector module is to employ a solar collector with a concentrator with a high concentration ratio, i.e., high solar collector area to thermal receiver area ratio.
For solar concentrators to work, solar trackers are required. Solar trackers follow the changes in relative position of the Sun in order to accomplish the concentration or focusing the Sun's radiation onto the thermal receiver aperture. Sun's movement is often described in two angular movements: “Hour Angle”, and also “Seasonal Angle” or “Declination Angle”. The Hour Angle describes the angular position of the Sun relative to an earth surface location due to Earth self-rotational daily periodic movement (i.e., Earth Spin); while the Declination (Seasonal) Angle describes the angular position of the Sun relative to an earth surface location due to the periodic movement of Earth-Sun rotational Axis relative to Earth Self-Spin Axis.
Traditionally, tracking of the movement of the sun is often done by rotating the entire optics-solar collector panel together with a solar receiver assembly in two axes, often called “moving target” tracking system. However, many of these solar concentrators comprise a single optical element per solar receiver resulting in a heavy system that must be rotated. As a result, tracking systems are typically expensive due to mechanics required provide the torque and acceleration desired to provide the desired movement.
Other solar concentrators comprise an array of optical element per solar receiver, individually moving to focus the Sun beam on a fixed solar receiver or target, usually called “heliostat” or “fixed target” system. In such configuration, for each optical element, the concentration ratio is either 1 or slightly higher than 1; however, many such optical elements project the sun light onto the same solar collector, and therefore resulting very high concentration ratio. Each such optical element has a different relative position and angle relative to the target, collectively forming a “Fresnel reflector”, i.e. arrays of small flat (or basically flat) mirrors forming a concave surface on a flat back plate. The optical cosine loss is large in such system (˜25%) since the Sun beam is not vertical to the reflecting mirrors in general. A heliostat of M×N optical elements usually require 2×M×N of independent moving axes (and therefore motors) to maintain focus (tracking) on a fixed target as Sun moves during the day and seasons, which is very expensive to implement.
A need exists, therefore, for improved techniques for tracking of the sun.